U.S. patent number 5,602,673 [Application Number 08/175,068] was granted by the patent office on 1997-02-11 for optical isolator without polarization mode dispersion.
This patent grant is currently assigned to Lucent Technologies Inc.. Invention is credited to Clarence B. Swan.
United States Patent |
5,602,673 |
Swan |
February 11, 1997 |
Optical isolator without polarization mode dispersion
Abstract
An optical isolator utilizes a pair of polarization selective
elements, as for example birefringent wedges, and all integral
Faraday rotator aligned therewith to perform optical signal
isolation without introducing the polarization mode dispersion
inherent in conventional polarization independent optical
isolators. Optical isolation is accomplished by passing a
forward-directed optical signal through a first birefringent
element which separates the optical signal into two orthogonal
states. The two orthogonal polarization states exchange identities
upon entering the second birefringent element from the Faraday
rotation element and are again deflected by the second birefringent
element so that they emerge from the second element parallel to
each other and, having traveled the same optical path length,
witllout any polarization mode dispersion. Both polarization states
of the reverse propagating optical signal are sufficiently
angularly deflected to avoid coupling with the optical signal
path.
Inventors: |
Swan; Clarence B. (Lower
Macungie Township, Lehigh County, PA) |
Assignee: |
Lucent Technologies Inc.
(Murray Hill, NJ)
|
Family
ID: |
22638732 |
Appl.
No.: |
08/175,068 |
Filed: |
December 29, 1993 |
Current U.S.
Class: |
359/281;
359/484.03; 359/489.09; 359/489.15 |
Current CPC
Class: |
G02B
6/4209 (20130101); G02F 1/093 (20130101) |
Current International
Class: |
G02F
1/01 (20060101); G02F 1/09 (20060101); G02B
6/42 (20060101); G02B 005/30 () |
Field of
Search: |
;359/280,281,282,283,484,494,495,496,497 ;372/703 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0054411 |
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Dec 1981 |
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EP |
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0533398A1 |
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Sep 1992 |
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EP |
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0176721 |
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Oct 1984 |
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JP |
|
0020016 |
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Jan 1986 |
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JP |
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0130920 |
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Jun 1986 |
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JP |
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0105908 |
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Apr 1989 |
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JP |
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1291212 |
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Feb 1990 |
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JP |
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0044310 |
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Feb 1990 |
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JP |
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4073712 |
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Mar 1992 |
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JP |
|
6011664 |
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Jan 1994 |
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JP |
|
Other References
Matsumoto, "Polarization-Independent Isolators for Fiber Optics",
Electronics and Communications in Japan, vol. 62-C, No. 7, 1979,
pp. 113-116. .
Compact optical isolator for fibers using birefringent wedges
Applied Optics/ vol. 21, No. 23/ 1 Dec. 1982 pp. 4296-4299. .
Polarization Independent Isolator Using Spatial Walkoff Polarizers
IEEE Photonics Technology Letters, vol. 1. No. 3, Mar. 1989 pp.
68-70..
|
Primary Examiner: Shafer; Ricky D.
Claims
What is claimed is:
1. An optical isolator adapted for receiving an applied
forward-directed optical signal, said optical isolator
comprising:
a first birefringent wedge having a first optical C-axis oriented
at a first optical axis angle, a first face, a second face and a
width defined between said first and second faces and varying from
a first width at a first end to a second width less than said first
width at a second end, said first wedge being oriented so that said
second end is oriented in a first direction and so that the applied
forward-directed optical signal enters said first wedge through
said first face and the forward-directed optical signal entering
the first wedge through said first face is divided into a first
polarization state and a second polarization state orthogonal to
said first polarization state and said first and second
polarization states exit said first wedge through said second
face;
a Faraday rotator disposed proximate said first birefringent wedge
for receiving said first and second polarization states exiting
said first wedge and from which said first and second polarization
states exit after passage through said Faraday rotator in which
said first and second polarization states undergo a nonreciprocal
45.degree. rotation; and
a second birefringent wedge having a first face, a second face, a
width defined between said first and second faces of the second
wedge and varying from a third width at a first end of the second
wedge to a fourth width less than said third width at a second end
of the second wedge, and a second optical C-axis oriented at a
second optical axis angle, said first and said second optical
C-axes being oriented relative to each other such that the
difference between said first and said second optical axis angles
equals 45.degree. in a rotational direction opposite said
nonreciprocal rotation of said Faraday rotator, said second wedge
being oriented so that said second end of the second wedge is
oriented in said first direction, and said second wedge being
disposed proximate said Faraday rotator so that said first and
second polarization states exiting said Faraday rotator enter said
second wedge through said first face of said second wedge and exit
said second wedge through said second face of said second
wedge;
said Faraday rotator being disposed between said first and second
birefringent wedges so that the first and second polarization
states of the applied forward-directed optical signal undergo an
exchange of identities in passing through the optical isolator in
that said second polarization state exiting said second
birefringent wedge is orthogonal to said second polarization state
exiting said first birefringent wedge and said first polarization
state exiting said second birefringent wedge is orthogonal to said
first polarization state exiting said first birefringent wedge, and
such that the first and second polarization states exit said second
birefringent wedge in parallel relation, each having travelled
through said optical isolator over respective optical paths of
substantially equal length, with substantially no polarization mode
dispersion.
2. The optical isolator of claim 1, wherein said first polarization
state comprises an ordinary wave and said second polarization state
comprises an extraordinary wave.
3. The optical isolator of claim 1 wherein said first wedge
includes at first edge, and said first optical C-axis of said first
wedge is oriented at an angle of approximately +22.5.degree.
relative to said first edge.
4. The optical isolator of claim 1, wherein said birefringent
wedges comprises TiO.sub.2.
5. The optical isolator of claim 1, wherein said first wedge and
said second wedge have the same shape.
6. The optical isolator of claim 1 wherein said first wedge
includes a first edge, and said first optical C-axis of said first
wedge is oriented at an angle of approximately 0.degree. relative
to said first edge.
7. The optical isolator of claim 1, wherein said first wedge abuts
said Faraday rotator.
8. The optical isolator of claim 7, wherein said second wedge abuts
said Faraday rotator.
9. The optical isolator of claim 1, wherein said second wedge abuts
said Faraday rotator.
10. The optical isolator of claim 1, wherein sai first wedge is
comprised of a material different than that comprising said second
wedge.
Description
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to an optical isolator which performs
optical isolation with substantially no polarization mode
dispersion.
2. Description of the Prior Art
Reflecting in optical systems often generate noise and optical
feedback which may degrade the performance of various system
components, particularly semiconductor lasers. Therefore, the
ability to optically isolate lasers and other sensitive components
from these reflections is critical to the performance of the
system. The Faraday effect in magneto-optic material enables the
construction of a unique non-reciprocal device capable of
performing the isolation function.
To reduce the insertion loss for coupled fibers, an isolator should
operate independent of the polarization state of the applied
signal. In general, a conventional optical isolator comprises a
45.degree. Faraday rotator encased within a bias magnet and
disposed between a pair of polarization selective means (e.g.
birefringent plates, or birefringent wedges) oriented at an angle
of 45.degree. to each other. The combination provides optical
isolation in the reverse direction because the Faraday rotation
means causes the two polarization states to switch identities as
they pass through the birefringent devices.
One arrangement for eliminating polarization dependence is
discussed in the article "Compact Optical Isolator For Fibers Using
Birefringent Wedges", by M. Shirasaki et al., 21 Applied Optics
4296-99 (1982). In particular, Shirasaki et al. utilize a pair of
birefringent wedges, located at the input and output of the Faraday
rotator, to separate an incident beam into orthogonal, linear
polarizations which travel independently through the isolator.
Signals passing through the isolator in the forward transmitting
direction will be essentially unaffected by the bireffingent wedges
and Faraday rotator whereas, in the reverse direction, both
polarization states undergo angular deviation so that neither
polarization state is coupled to the input signal path of the
isolator. Although the Shirasaki et al. arrangement, and other
commercially available isolators, may be polarization independent,
they can exhibit polarization mode dispersion in that the
propagation time of a ray through the birefringent material is a
function of its polarization state (i.e. extraordinary polarization
state or ordinary polarization state). In particular, the
birefringent material will have a different refractive index for
each polarization state. As a result, a net dispersion (i.e.
propagation delay between polarization states) that can be on the
order of picoseconds will exist as the rays emerge from the
isolator.
The net polarization mode dispersion At is calculated according to
the formula: ##EQU1## where L is the total path length in the two
wedges, c is the speed of light in free space, n.sub.o is the
refractive index seen by the ordinary ray, and n.sub.e is the
refractive index seen by the extraordinary ray.
For a standard Type 25 or Type 26 isolator manufactured by
AT&T, which utilizes rutile material (n.sub.o =2.454 and
n.sub.e =2.710), the net dispersion becomes: ##EQU2## For some
applications, such as cascaded amplifiers in undersea systems, this
degree of dispersion may present a serious problem. Accordingly,
various techniques have been attempted in order to compensate for
this dispersion through the inclusion of one or more additional
compensation elements added to the isolator.
A need nevertheless remains in the art for a simple and effective
optical isolation means that introduces substantially no
polarization mode dispersion and which therefore requires no
polarization mode dispersion correction.
SUMMARY OF THE INVENTION
The need remaining in the prior art is addressed and successfully
satisfied by the present invention which provides an optical
isolator that requires no supplemental polarization mode dispersion
correction.
In a preferred embodiment of the invention, an optical isolator
comprises a Faraday rotation means interposed between two
polarization selective means which, in the most preferred case, are
a pair of rutile birefringent wedges. The birefringent wedges split
the optical signal into orthogonal polarization states, i.e.
ordinary and extraordinary signals, and their optical axes (C-axes)
are oriented such that, in combination with the 45.degree.
polarization rotation (nonreciprocal) associated with the Faraday
rotation means, the polarization states of a signal propagating
through the isolator in the forward direction exchange identities
as they pass through the birefringent devices.
The optical axis (C-axis) of the second birefringent device is
rotated counterclockwise by an angle of 45.degree. with respect to
the optical axis of the first birefringent device. The interposed
Faraday rotator rotates the transmitted light signal by an angle of
45.degree. in the clockwise direction. As a result, the
polarization states of a signal propagating through the isolator in
the forward direction exchange identities as they enter the second
birefringent device from the Faraday rotator (i.e. the ordinary ray
emerging from the first birefringent device will emerge from the
second birefringent device as the extraordinary ray and vice
versa). As a result, the ordinary and extraordinary rays exit the
second birefringent device parallel to one another with
substantially no polarization mode dispersion. Although the optical
isolator of the present invention introduces a net beam deflection,
since the rays are parallel, they can both be simultaneously
focused on a fiber. Compensation for this deflection may be readily
provided by means known in the art and not forming a part of the
present invention such, for example, as by using beveled graded
index lenses or by appropriately aligning or tilting the input and
output fiber and lens assemblies.
The optical isolator of the present invention provides the
requisite optical isolation in the reverse direction since the
Faraday rotation means causes the two polarization states to retain
their identities in passing through the birefringent devices (i.e.
the reverse directed ordinary ray retains its identity passing from
the second birefringent device through the Faraday rotator and into
the first birefringent device, as does the extraordinary ray). The
result is that the reverse directed ordinary ray and extraordinary
ray experience different total angular deviations which are both
directed away from the signal path of the isolator. Consequently,
neither polarization state is coupled to the input signal path.
In further accordance with a preferred embodiment of the present
invention, the lengths of the paths of the ordinary ray and the
extraordinary ray are equalized such that they exit the isolator
with substantially no net dispersion--i.e. with substantially no
propagation delay difference between the two polarization
states.
Other objects and features of the present invention will become
apparent from the following detailed description considered in
conjunction with the accompanying drawings. It is to be understood,
however, that the drawings are designed solely for purposes of
illustration and not as a definition of the limits of the
invention, for which reference should be made to the appended
claims.
DETAILED DESCRIPTION OF THE DRAWINGS
In the drawings, wherein like reference characters denote similar
elements throughout the several views:
FIG. 1 illustrates an exemplary polarization dispersion optical
isolator formed in accordance with the teachings of the present
invention;
FIG. 2 illustrates an exemplary packaged optical isolator of the
present invention;
FIG. 3 illustrates an alternative embodiment of a packaged optical
isolator of the present invention; and
FIG. 4 depicts an optical isolator formed in accordance with the
teachings of the present invention, with the directional paths of
the light rays of interest passing through the isolator
conceptually illustrated.
DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS
FIG. 1 illustrates a simplified drawing of a first embodiment of
the optical isolator 10 of the present invention. It should be
understood and apparent to those skilled in the art that, in actual
operation, such an isolator additionally requires a niagnet or
magnets appropriately positioned about the illustrated structure
and may also include correction elements, all as discussed
hereinbelow in association with FIGS. 2 and 3. For the sake of
clarity, however, these other components are not shown in FIG. 1
and are not necessary for understanding the operation of the
inventive isolator arrangement.
Referring to FIG. 1, the isolator 10 includes a Faraday rotator 12
(fabricated, for example, of Yttrium Iron Garnet (YIG) crystal or
bismuth-substitutecl YIG) disposed between a pair of polarization
selective means 14, 16. The polarization selective means 14, 16 may
be fabricated of, for example, rutile, lithium niobate, calcite or
other birefringent material and comprise a pair of birefringent
wedges. The optical axis (C-axis) C.sub.14 of birefringent device
14 is oriented at an angle .beta..sub.1 with respect to edge 19 of
wedge 14, while the C-axis C.sub.16 of birefringent device 16 is
oriented at an angle .beta..sub.2 with respect to edge 21 of wedge
14. Birefringent device 16 is selected and oriented so that the
C-axis C.sub.16 of birefringent device 16 is rotated
counterclockwise by an angle of 45.degree. relative to the C-axis
C.sub.14 of birefringent device 14. For example, for the preferred
embodiment shown in FIG. 1, because edge 19 and edge 21 are
parallel to one another and because the optical isolator is linear
in that the wedges 14, 16 and the Faraday rotator 12 are linearly
aligned, the angular separation between the C-axis C.sub.14 of
wedge 14 and the C-axis C.sub.16 of wedge 16 may be obtained by
merely adding the values of .beta..sub.1 and .beta..sub.2 as shown
in FIG. 1. Thus, for example, .beta..sub.1 and .beta..sub.2 may
both be equal to +22.5.degree., as shown in FIG. 1, .beta..sub.1
may be equal to 0.degree. and .beta..sub.2 may be equal to
+45.degree., or .beta..sub.1 may equal -10.degree. and .beta..sub.2
may equal +55.degree.. Faraday rotator 12 is a device well-known in
the art and is selected to provide a nonreciprocal clockwise
rotation of 45.degree. of the polarization of an optical signal
passing therethrough. Such a rotation corresponds to a clockwise
rotation with respect to the forward propagating direction of the
optical signal. Thus, as discussed in greater detail below, because
of the counterclockwise 45.degree. rotation of the optical axis of
the second wedge 16 relative to that of the first wedge 14 in
combination with a clockwise 45.degree. rotation by the Faraday
rotator 12, a transmitted optical signal will pass relatively
unimpeded in the forward or transmitting direction through the
isolator 10, while a reverse propagating, reflected optical signal
will be sufficiently deflected by the isolator 10 that it will not
re-enter the optical signal path.
The two polarization states (extraordinary and ordinary) of a
transmitted optical signal exhibit different transit times as they
propagate through each of the birefringent wedges 14 and 16.
Preferably, the birefringent material is rutile (i.e. crystalline
titanium dioxide (TiO.sub.2)) although other known materials, such
as lithium niobate and calcite, may be used. For rutile, the
refractive index n.sub.e for an extraordinary ray is slightly
greater than the refractive index n.sub.o for an ordinary ray
traveling through the same material. The refractive index of a
birefringent material is defined as the ratio of the signal's
velocity in free space with respect to the velocity in the
birefringent material. For rutile wedges utilized with an input
lightwave at .lambda.=1.5 .mu.m, n.sub.e is approximately 2.710 and
n.sub.o is approximately 2.454.
The first polarization selective means or birefringent wedge 14 has
a surface 18 beveled at an angle .alpha..sub.1 with respect to
planar surface 22 and separates an incoming or applied optical
signal into an ordinary ray and an associated orthogonal
extraordinary ray. The ordinary and extraordinary rays will be
deflected by the wedge 14 by different amounts in accordance with
the respective n.sub.o and n.sub.e of the polarization selective
means 14.
A Faraday rotator 12 is positioned so that it is juxtaposed or
adjacent to the polarization selective means 14 so that the planar
surface 22 is adjacent the Faraday rotator 12. The Faraday rotator
12 is used to rotate the ordinary and extraordinary rays emerging
therefrom by 45.degree. and to ensure that the ordinary and
extraordinary rays passing in the reverse direction will be
deflected such that neither will couple with the input signal
path.
A second polarization selective means or birefringent wedge 16,
having a surface 20 beveled at an angle .alpha..sub.2 with respect
to planar surface 23, is positioned so that it is following and is
juxtaposed or adjacent to the Faraday rotator 12. Birefringent
wedge 16 further deflects the light signal in accordance with the
refractive indices n.sub.e and n.sub.o of the second means 16. The
orientation of the C-axis C.sub.16 of the second wedge 16 relative
to the C-axis C.sub.14 of the first wedge 14 combined with the
45.degree. rotation caused by the Faraday rotator 12 cause the
extraordinary ray leaving the first wedge 14 to be treated as an
ordinary ray upon entering the second wedge 16 and cause the
ordinary ray leaving the first wedge 14 to be treated as an
extraordinary ray upon entering the second wedge 16. Consequently,
the two polarization states of the optical signal exchange
identities and the forward-directed ordinary and extraordinary rays
emerging from the second means 16 will thus have experienced a net
deflection relative to the input optical signal but will be
parallel to each other with substantially no net polarization mode
dispersion. Polarization mode dispersion is essentially eliminated
by utilizing the resulting exchange of identities of the
extraordinary and ordinary rays thereby equalizing the time
required for the extraordinary and ordinary rays to traverse the
isolator 10. As a consequence, there is substantially no net time
delay of the extraordinary ray relative to the ordinary ray when
they emerge from the birefringent wedge 16, i.e. there is
substantially no polarization mode dispersion introduced between
the input and output ends of the isolator 10.
The ability of the optical isolator of the present invention to
isolate an incident beam from a reflected beam can be demonstrated
mathematically. Assuming that an incident beam enters the isolator
at an angle of 0.degree., that the two birefringent wedges 14, 16
are made of the same material, and that any spaces between an
adjacent isolator elements is air filled with a refractive index of
1.0, the net deflection in the forward direction between the input
and output rays by the isolator 10 of the present invention may be
calculated as follows. For the ray which leaves wedge 14 as the
ordinary ray and becomes the extraordinary ray upon entering the
second wedge 16 after Faraday rotation, the deflection is
calculated (using the small angle approximation) as:
Similarly, for the ray which leaves wedge 14 as the extraordinary
ray and becomes the ordinary ray upon entering the second wedge 16
after Faraday rotation, the deflection is calculated (using the
small angle approximation) as:
Where the two wedges 14, 16 have the same bevel angle, such that
.alpha..sub.1 =.alpha..sub.2, the total deflection of the rays
emerging from the second polarization selective means 16 with
respect to the input optical signal may be approximated as:
For the reverse propagated rays, assuming an initial reverse input
ray angle of 0.degree., the deflection of the reverse propagated
ray originating as the ordinary ray is calculated (using the sinall
angle approximation) as:
Similarly, the deflection of the reverse propagated ray originating
as the extraordinary ray is calculated (using the sinall angle
approximation) as:
However, for an incident beam angle of 0.degree., the initial
reverse input ray angle will not be at 0.degree., it will be
.PHI..sub.total. Therefore, the deviation of the reverse propagated
ordinary ray is calculated as:
and the deviation of the reverse propagated extraordinary ray is
calculated as:
After substituting Equations 4 and 5 into Equation 7, and after
substituting Equations 4 and 6 into Equation 8, it can be seen
that:
Since, by definition n.sub.o is not equal to n.sub.e, neither
.PHI..sub.o total.sup.Rev nor .PHI..sub.e total.sup.Rev is equal to
zero, the assumed incident beam angle. Thus, neither reverse
propagated ray will couple to the initial signal path. This effect
can be demonstrated by selecting typical values for the variables
in the equations.
Assuming an incident beam angle of 0.degree., for identical wedges
14, 16 where the bevel angle (.alpha..sub.1=.alpha..sub.2 =.alpha.)
is selected to be 3.5.degree. and where the material comprising the
wedges 14, 16 is selected to be rutile (n.sub.e approximately 2.710
and n.sub.o approximately 2.454), the total deflection,
.PHI..sub.total, from input to output will be approximately
11.07.degree.. For an initial reverse propagated ray angle of
0.degree., the deflection, .PHI..sub.o.sup.Rev, of the reverse
propagated ray originating as the ordinary ray will be
approximately 10.18.degree. and the deflection,
.PHI..sub.e.sup.Rev, of the reverse propagated ray originating as
the extraordinary ray will be approximately 11.97.degree.. Thus, in
the example
Since neither of these angles is equal to zero, the assumed
incident beam angle, the reflected ordinary and extraordinary rays
do not couple with the input signal path. A typical optical fiber
used in conjunction with the optical isolator of the present
invention has a core diameter of approximately 0.8 .mu.m. When such
a typical fiber is used, a deflection of less than 1.degree. of the
reflected light signal is sufficient to prevent coupling with the
incident beam to provide the requi site optical isolation.
Furthermore, utilizing an optical isolator constructed in
accordance with this embodiment of the present invention and all
input beam angle of approximately 5.54.degree., the deflected
ordinary and extraordinary ray will emerge from the second means 16
at approximately 5.54.degree.. Thus, by employing beveled graded
index lenses, which are well known and form no part of the present
invention, an in-line assembly is possible.
FIG. 2 illustrates an exemplary packaged isolator 40 constructed in
accordance with the present invention. First polarization selective
means or birefringent wedge 14 and second polarization selective
means or birefringent wedge 16 are adjacent or juxtaposed to the
Faraday rotor 12 so that the planar surface 22 of wedge 14 and the
planar surface 23 of wedge 16 both abut or contact the Faraday
rotor 12. An optically transparent and anti-reflective adhesive or
other material is preferably used to join the Faraday rotator 12 to
the two wedges 14, 16. Alternatively, as shown in FIG. 3, the first
polarization selective means or birefringent wedge 14 and second
polarization selective means or birefringent wedge 16 may be
mounted so that there are gaps 30, 32 between surfaces 22, 23 of
respective wedges 14, 16 and the Faraday rotator 12 or so that
there is a gap between one wedge 14 or 16 and the Faraday rotator
12. Alternatively, as shown in FIG. 3, both wedges 14, 16 may be
rotated 180.degree. so that the beveled surfaces 18, 20 are
adjacent the Faraday rotator 12 and so that gaps 30, 32 are
disposed between the wedges 14, 16 and the Faraday rotator 12. When
there is a gap between the Faraday rotator 12 and either or both
wedges 14, 16, the adjacent surfaces are preferably coated with an
optically transparent and anti-reflective material. Referring to
FIG. 2, wedges 14, 16 and Faraday rotator 12 are disposed within a
bore in a magnet 42 which provides a magnetic field to the
magneto-optic material comprising the Faraday rotator 12 thereby
causing the requisite 45.degree. rotation of the ordinary and
extraordinary rays. The magnet 42 is fixed or secured within an
outer housing or package 44. Alternatively, the position of the
magnet 42 relative to the Faraday rotator 12 may be adjustable so
that the Faraday rotator 12 may be fine tuned. Such an adjustable
magnet is disclosed in U.S. Pat. No. 5,111,330 which is
incorporated herein by reference. Alternatively, as shown in FIG.
3, magnet 42 may be comprised of two magnets 42a and 42b, the
positions of which may be fixed or adjustable. An input
transmission element 46 is mounted and appropriately positioned
within a fixture 48 attached to the input end of the outer package
44 to form a collimated beam for reception by first polarization
selective means 14. Similarly, an output transmission element 50 is
mounted and appropriately positioned within a fixture 52 that is
attached to the opposite or output end of the outer package 44 to
form a focused beam from second polarization selective means
16.
Packaged isolator 40 may also incorporate additional elements or
means to form a focused beam or to compensate for the net beam
deflection for example by using beveled graded index lenses and/or
appropriately aligning or tilting the input and output fiber and
lens assemblies.
In FIG. 4, a representative diagram conceptually illustrates the
direction of the light rays passing through the optical isolator 10
of the instant invention.
Although packaged isolator 40 has been shown and described as an
in-line assembly, arrangements otller than in-line may also be
used. In such embodiments, the two wedges 14, 16 and the Faraday
rotator 12 are not positioned in a straight line, but instead one
or more light defracting or reflecting elements are interposed
between or among the elements. Consequently, the light signal will
travel, for example, in a curved or L-shaped path and the edge 19
of the first birefringent wedge 14 will not be parallel to the edge
21 of the second birefringent wedge 16. In these alternative
embodiments of the present invention, the required relative
orientations of the two wedges 14, 16 is selected so that the
required exchange of polarization states occurs as the light signal
enters the second birefringent device 16 alter leaving the Faraday
rotator 12. It will be recognized and understood that similar
results may also be obtained utilizing materials other than those
which are currently preferred and herein disclosed for forming the
polarization selective means 14, 16. Where such other materials are
employed, there must be a sufficient difference in the refractive
indices n.sub.e and n.sub.o of the material, and each polarization
selective means should have appropriate respective wedge angles,
.alpha..sub.1 and .alpha..sub.2, as, by way of example, seen in
Equations 2 through 9, to ensure that the reverse propagated rays
do not couple into the signal path. Similarly, where wedge 14 is
comprised of a material different from wedge 16, the shapes of the
wedges are selected to take into account their respective
refractive indices to ensure that the path lengths travelled by the
two polarization states of the optical signal are substantially
equal, thereby substantially eliminating polarization mode
dispersion. Additionally, although the optical axis of the second
birefringent wedge 16 is disclosed as being rotated by 45.degree.
in a counterclockwise direction relative to the optical axis of the
first birefringent wedge 14 and the Faraday rotator as rotating the
signal 45.degree. in a clockwise direction, both of these
directions of rotation may alternatively be reversed.
Thus, while there have been shown and described and pointed out
fundamental novel features of the invention as applied to preferred
embodiments thereof, it will be understood that various omissions
and substitutions and changes in the form and details of the
devices illustrated, and in their operation, may be made by those
skilled in the art without departing from the spirit of the
invention. It is the intention, therefore, to be limited only as
indicated by the scope of the claims appended hereto.
* * * * *